1. Field of the Invention
The present invention relates to a high frequency circuit, an amplifier, a mixer, an oscillator, and a radio circuit using them, and more particularly to a high frequency differential circuit, a differential amplifier, a differential mixer, a differential oscillator, and a radio circuit using them.
2. Description of the Background Art
In a differential amplifier for use in a high frequency circuit, it is necessary to enhance its reverse isolation characteristic in order not to reduce a power amplification factor. Enhancing a reverse isolation as used herein means to suppress a so-called negative feedback effect of a signal fed back from an output to an input which reduces the level of an input signal. Conventionally, configuring a differential amplifier with a cascode amplifier is known to be effective in enhancing the reverse isolation characteristic (see Takeshi Yasunaga et al., “A Fully Integrated PLL Frequency Synthesizer LSI For Mobile Communication System”, 2001 IEEE Radio Frequency Integrated Circuit Symposium, IEEE Service Center, USA, May 20, 2001, vol. 1, pp. 65–68).
FIG. 21 is a circuit diagram of a conventional differential amplifier including cascode amplifiers. In FIG. 21, the differential amplifier includes transistors 91a and 91b, transistors 92a and 92b, a resistor 93, load resistors 94a and 94b, a constant-current supply 95, a positive supply 96, a ground 97, and a bias supply 98.
The transistors 91a and 91b constitutes a differential pair for amplification. Signals in reverse phase with each other are inputted into bases of the transistors 91a and 91b. 
An emitter of the transistor 92a is connected to a collector of the transistor 91a, thereby forming a cascode amplifier. Similarly, an emitter of the transistor 92b is connected to a collector of the transistor 91b, thereby forming a cascode amplifier.
The resistor 93 is connected between emitters of the transistors 91a and 91b. The constant-current supply 95 is connected between a tap of the resistor 93 and the ground 97.
Bases of the transistors 92a and 92b are connected together. The bias supply 98 is connected between the bases of the transistors 92a and 92b in order to supply a constant bias.
A collector of the transistor 92a is connected to one end of the load resistor 94a. Similarly, a collector of the transistor 92b is connected to one end of the load resistor 94b. The positive supply 96 is connected to the other ends of the load resistors 94a and 94b. Connected between the transistor 92a and the load resistor 94a is an output terminal 99a. Connected between the transistor 92b and the load resistor 94b is an output terminal 99b. Signals in reverse phase with each other are outputted from the output terminals 99a and 99b. 
The transistors 92a and 92b reduce variation of collector voltages of the transistors 91a and 91b, which constitute a differential pair for amplification, due to a signal current. Accordingly, it is possible to minimize the miller effect of the transistors 91a and 91b, thereby enhancing the reverse isolation characteristic of the differential amplifier.
As described above, by providing cascode amplifiers in a differential amplifier, it is made possible to enhance the reverse isolation characteristic of the differential amplifier, whereby it is possible to prevent gain from decreasing in a high frequency region.
However, even in the differential amplifier including the cascode amplifiers, the reverse isolation characteristic thereof is reduced in a frequency region above 100 MHz mainly due to a parasitic capacitance present between input and output ends.
For example, in the case of a differential amplifier included in a mobile telephone as an RF amplifier connected between an antenna and a mixer, if the reverse isolation characteristic of the RF amplifier is reduced, a signal derived from the mixer leaks into the antenna side via the RF amplifier. Also, in the case of a differential amplifier used as a local amplifier connected between a local oscillator and a mixer, if the reverse isolation characteristic of the local amplifier is reduced, a signal derived from the mixer leaks into the local oscillator via the local amplifier, deteriorating a noise characteristic of the local oscillator.
Accordingly, there is no satisfactory conventional differential amplifier in terms of the reverse isolation characteristic in a high frequency region.
The problem as described above is not limited to the differential amplifier, and also occurs in high frequency differential circuits, such as a differential mixer, a differential oscillator, etc., which include the differential amplifier.